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TPS54302

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SLVSDG6A – MAY 2016 – REVISED MAY 2016

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Copyright © 2016, Texas Instruments Incorporated

7.3 Feature Description

7.3.1 Fixed-Frequency PWM Control

The device uses a fixed-frequency, peak current-mode control. The output voltage is compared through external

resistors on the FB pin to an internal voltage reference by an error amplifier. An internal oscillator initiates the

turn on of the high-side power switch. The error amplifier output is compared to the current of the high-side

power switch. When the power-switch current reaches the error amplifier output voltage level, the high side

power switch is turned off and the low-side power switch is turned on. The error amplifier output voltage

increases and decreases as the output current increases and decreases. The device implements a current-limit

by clamping the error amplifier voltage to a maximum level and also implements a minimum clamp for improved

transient-response performance.

7.3.2 Pulse Skip Mode

The TPS54302 is designed to operate in pulse skipping mode at light load currents to boost light load efficiency.

When the peak inductor current is lower than 500 mA typically, the device enters pulse skipping mode. When the

device is in pulse skipping mode, the error amplifier output voltage is clamped which prevents the high side

integrated MOSFET from switching. The peak inductor current must rise above 500 mA and exit pulse skip

mode. Since the integrated current comparator catches the peak inductor current only, the average load current

entering pulse skipping mode varies with the applications and external output filters.

7.3.3 Error Amplifier

The device has a trans-conductance amplifier as the error amplifier. The error amplifier compares the FB voltage

to the lower of the internal soft-start voltage or the internal 0.596-V voltage reference. The transconductance of

the error amplifier is 240 µA/V typically. The frequency compensation components are placed internal between

the output of the error amplifier and ground.

7.3.4 Slope Compensation and Output Current

The device adds a compensating ramp to the signal of the switch current. This slope compensation prevents

sub-harmonic oscillations as the duty cycle increases. The available peak inductor current remains constant over

the full duty-cycle range.

7.3.5 Enable and Adjusting Undervoltage Lockout

The EN pin provides electrical on and off control of the device. When the EN pin voltage exceeds the threshold

voltage, the device begins operation. If the EN pin voltage is pulled below the threshold voltage, the regulator

stops switching and enters the low-quiescent (IQ) state.

The EN pin has an internal pullup-current source which allows the user to float the EN pin to enable the device. If

an application requires control of the EN pin, use open-drain or open-collector output logic to interface with the

pin.

The device implements internal undervoltage-lockout (UVLO) circuitry on the VIN pin. The device is disabled

when the VIN pin voltage falls below the internal VIN UVLO threshold. The internal VIN UVLO threshold has a

hysteresis of 480 mV.

If an application requires a higher UVLO threshold on the VIN pin, then the EN pin can be configured as shown

in Figure 13. When using the external UVLO function, setting the hysteresis at a value greater than 500 mV is

recommended.

The EN pin has a small pull-up current, Ip, which sets the default state of the pin to enable when no external

components are connected. The pull-up current is also used to control the voltage hysteresis for the UVLO

function because it increases by Ih when the EN pin crosses the enable threshold. Use Equation 1 and

Equation 2 to calculate the values of R4 and R5 for a specified UVLO threshold.